The present disclosure relates to apparatuses having a function of capturing moving images, such as digital cameras, camera phones, etc., and moving image compressing (encoding) techniques which are employed when image contents are produced or used.
In recent years, moving image compressing (encoding) techniques, such as the moving picture experts group (MPEG) etc., which can perform highly efficient encoding, have been rapidly put into practice and widely used in camcorders, mobile telephones, etc.
In encoding techniques, such as MPEG etc., various encoding modes are defined in the standards. For example, in MPEG-4, there are two modes: “intra-encoding mode” in which encoding is performed with respect to only an image which is contained in a frame of interest (hereinafter referred to as a target image); and “inter-encoding mode” in which an image region having a strong correlation with a target image is searched for (hereinafter referred to as a motion search) in a frame which has already been encoded (hereinafter referred to as a reference frame), and encoding is performed with respect to only a difference value between an image after the motion search (hereinafter referred to as a motion-compensated image) and the target image.
In the inter-encoding mode, discrete cosine transform (DCT) and quantization are applied to the difference image between an input image and a motion-compensated image, and thereafter, a combination of the resulting data, i.e., DCT coefficients, and a motion vector indicating the position of an image to be referenced in a reference frame are variable-length encoded to generate a stream (see the MPEG-4 standard).
More specifically, in the inter-encoding mode, if the input image and the motion-compensated image completely match, i.e., the difference image is zero, zero is input with respect to DCT and quantization, and zero is output as the result. If the difference image is not zero, but is not very large, zero may be output as the result of quantization. Thus, when the output result is zero, DCT and quantization are not performed, i.e., skipped, and the result is replaced with zero, which is then output, resulting in higher speed (see Japanese Patent Publication No. 2004-328799.